1. Field of the Invention
The present invention relates to a discharge unit for an excimer or molecular gas laser, particularly having a narrow discharge width and aerodynamic gas flow.
2. Discussion of the Related Art
Pulsed gas discharge lasers, emitting in the deep ultraviolet region (DUV) and/or vacuum ultraviolet region (VUV), are important tools for a wide range of industrial applications. For example, microlithography applications currently use a line narrowed excimer laser (e.g., ArF, KrF, XeCl, KrCl or XeF) or a molecular fluorine (F2) laser having high efficiency and stability at high repetition rates (e.g., 1000 Hz or more).
An electrode chamber design and electrode configuration of a conventional discharge unit are illustrated in FIG. 1. The electrode chamber of FIG. 1 houses a pair of elongated main electrodes 2, 4. The main electrodes 2, 4 are separated by a gap or discharge area 6 through which a gas mixture is flowed. A set of high voltage capacitors or xe2x80x9cpeakingxe2x80x9d capacitors Cp is preferably positioned as close as possible to the main discharge electrodes 2, 4, and as uniformly as possible over the length of the electrodes 2, 4. One or two or more preionization units 10 are used to preionize the gas mixture in the discharge area 6 prior to the main discharge.
One of the main electrodes, in this case electrode 2, is connected to a pulsed high voltage generator. The high voltage generator typically includes a thyratron or a solid state switch for providing a fast and powerful charge to the peaking capacitors 8 up to the electrical breakdown voltage of the gas discharge gap 6. The other main electrode 4 is usually connected to ground potential. Fast and powerful discharge of the peaking capacitors Cp, followed by electrical breakdown of the active laser gases in the gas mixture provides the necessary pumping of the gas mixture.
The peaking capacitors in both cases are disposed outside of the electrode chamber (that is not necessary, but common, because it easily avoids exposure of the peaking capacitors to the aggressive halogen gas). One of the main discharge electrodes, the ground electrode, is connected directly to the metal body of the electrode chamber. The other or high voltage electrode is connected to the peaking capacitors and is separated from the grounded metal body of the electrode chamber by means of a dielectric (e.g., ceramic) insulator.
The gas mixture is characterized as being strongly electronegative and maintained at an elevated pressure (e.g., a few bars). The gas mixture for an excimer laser includes an active rare gas such as krypton, argon or xenon, a halogen containing species such as fluorine or HCl, and a buffer gas such as neon or helium. A molecular fluorine laser includes molecular fluorine and a buffer gas such as neon and/or helium.
A typical preionization arrangement includes two preionization units 10 each including a conducting electrode inside a dielectric tube. The preionization units 10 are connected to a pulsed high voltage source and preionize the gas mixture by forming a uniform surface glow discharge. The preionization units 10 are typically positioned in the vicinity of the discharge area 6 between the main electrodes 2, 4 and provide an initial ionization of the laser gas during the charging of the peaking capacitors Cp by the high voltage pulsed generator. UV-preionizers typically include arrays of electrical sparks, sometimes stabilized by dielectric surfaces, or other configurations of barrier or corona discharge sources. Soft x-ray radiation sources are also sometimes used.
Examples of preionization arrangements which could be used for UV-preionization are shown in FIGS. 2a and 2b. FIG. 2a shows a corona preionization arrangement including two corona units 10a. Each corona unit 10a shown includes an cylindrical electrode 16 surrounded by a dielectric tube 18. An external electrode 20 a provides a potential difference for each preionization unit 10a. The UV radiation emitted by the preionization units 10a preionizes gaseous components within the discharge area 6.
FIG. 2b shows a cross section of a UV-spark preionization arrangement wherein the preionization units 10b include separate pins 22 surrounded by dielectrics 24. These pins 22 are fed-through the chamber and connected to a pulsed power source outside the chamber. A plurality of spark gaps 26 are formed due to a potential difference between an electrode 20b in proximity to the pins 22 and produces preionization of the gas in the discharge area 6.
Besides the discharge unit having a pulser circuit and a laser tube including an electrode chamber such as that illustrated in FIG. 1, the laser tube of the discharge unit further includes a gas vessel 11 having a gas flow system or blower 12 and a heat exchanger 14 as illustrated in FIG. 3. A vane 15 is also shown extending from the blower 12 generally to the electrode 4 of the discharge chamber. The blower 12 forces the gas to flow generally as indicated by the arrows in FIG. 3. The gas mixture is naturally heated as it is excited by the electrical discharge in the discharge area 6. The heat exchanger 14 cools the heated gas after it exits the electrode chamber. The portion of the gas mixture which participates in a laser pulse is replaced by fresh gas before the next laser pulse occurs. Although not shown, a gas supply unit also typically supplies fresh gas to the system from outside gas containers to replenish each of the components of the gas mixture. In particular, halogen containing gas is typically supplied because the halogen concentration in the gas mixture tends to deplete rapidly during operation, while it is desired to maintain a constant or near constant halogen concentration in the gas mixture. Means for releasing some of the gas mixture is also typically provided so that the gas pressure can be controlled and to expel contaminated gases.
Above, various components of a pulsed gas discharge laser such as an excimer or molecular laser have been discussed with respect to their design and arrangement within the electrode chamber. The design and placement of the electrode chamber itself relative to the gas vessel 11, the placement of the peaking capacitors Cp, and the insulation of the high voltage electrode 2 are further considerations in effective discharge unit design. Examples of laser designs are illustrated in cross-sectional views at the FIGS. 4a and 4b. 
The discharge unit illustrated at FIG. 4a includes a dielectric frame or one or two or more dielectric insulators 28 (see Industrial Excimer Lasers: Fundamentals, Technolgy and Maintenance, Dirk Basting, Ed., 2nd edition (1991); Litho laser tube of Lambda Physik, GmbH). Each dielectric insulator 28 is mechanically connected to the gas vessel 11 that is connected to the grounded discharge electrode 4. The dielectric frame or insulator(s) 28 electrically isolate the high voltage electrode 2. That is, the roof 31 connected to the high voltage electrode 2 is insulated from the grounded main electrode 4 by the dielectric insulator(s) 28.
Where the electrode chamber, e.g., as shown in FIG. 4a, meets the gas vessel 11, an arrangement 30 of conducting ribs are connected electrically to the grounded electrode 4. The rib arrangement 30 of the discharge unit includes several rectangular ribs 32 separated by openings to permit gas flow from the gas vessel 11 into the electrode chamber and into the discharge area 6. The relationship between the rectangular 32 and the opening separating them are illustrated at FIG. The ribs 32 serve as low inductive current conductors in the discharge circuitry. A lower inductivity of the discharge electrical current loop is advantageous as better matching may be provided between the wave impedance of the electrical discharge loop and the gas discharge impedance.
The discharge unit of FIG. 4a advantageously allows the discharge loop to exhibit a characteristically low inductivity. However, the gas flow through the discharge area 6, and especially near the grounded electrode 4, has a high curvature producing turbulences that complicate the gas exchange in the discharge area 6.
Another consideration arises with respect to the nearly rectangular interior shape of the electrode chamber. Powerful and symmetric energy dissipation in the gas discharge area 6, particularly when the system is operating at a high repetition rate, can lead to acoustical resonances and amplification of the level of standing acoustical waves. Modulation of the gas density by the acoustical disturbances can have an adverse influence on the uniformity of the gas discharge and ultimately on significant laser output parameters.
One way to reduce the level of these acoustical disturbances is to introduce acoustical dampers into the field of the acoustical waves. These dampers may be used as obstacles for the acoustical waves. However, the dampers can also have an adverse influence on the uniformity of the gas flow. In addition, the dampers would have large surface areas which are subject to attack by aggressive halogens in the gas mixture.
FIG. 4b shows an alternative discharge unit design to that illustrated at FIG. 4a (see U.S. Pat. Nos. 4,891,818 to Levatter and 5,771,258 to Morton et al.). A dielectric insulator plate 33 separates the high voltage electrode from the metal walls of the electrode chamber. The main electrodes 2, 4 are immersed in the gas flow vessel 11. Electrical current return bars similar to the rectangular ribs 32 of the arrangement of FIG. 4a may once again cross the gas flow and shorten the discharge loop from the grounded discharge electrode 4 to the walls of the laser tube. The gas exchange conditions are improved over those discussed above with respect to the arrangement of FIG. 4a. 
The improved gas exchange conditions provided by the arrangement of FIG. 4b are advantageous because satisfactory laser operation may be achieved at lower gas flow rates, and strong and uniform gas flow permits satisfactory operation at higher repetition rates (see U.S. Pat. No. 5,247,534 to Muller-Horsche, assigned to the same assignee as the present invention, and hereby incorporated by reference). However, the connection of the high voltage electrode 2 via the dielectric plate 33 implies the use of a plurality of concentrated feedthroughs 34. This gives rise to an undesirably higher inductivity of the electrical discharge current loop.
FIG. 4f shows an alternative design. The insulators 128 shown in FIG. 4f conform with the gas flow.
Another consideration of discharge unit design is the main electrodes 2, 4 themselves. Features of the main electrodes 2, 4 including their size, shape and proximity to each other and to other elements within the electrode chamber such as the preionization units determine important discharge conditions such as the shape and uniformity of the static electrical field in the discharge area 6 and the width of the discharge area 6.
In line narrowed lasers, used as illuminating sources for microlithography, some additional considerations amplify the desirability of minimizing the discharge width. One of these is the design of the resonator assembly. The discharge width should be reduced to a value commensurate with the effective aperture size of the line narrowing resonator. For example, an effective aperture of a linewidth narrowing resonator might be on the order of 3 to 4 mm or less, and is typically around 2 mm. Thus, the discharge width should be comparable to or less than this 3 to 4 mm specification.
A narrower discharge width is also more suitable for laser operation at higher repetition rates (e.g., 1 kHz or more). Yet another advantage to having a narrow discharge width is that the exchange of gases in the discharge area is simplified.
In combination with design considerations involving the static field and discharge width parameters as discussed above, the electrodes 2, 4 should have a minimized width to provide the most compact and least inductive design possible of the gas discharge electrical circuit. Analytical expressions for the shapes of the electrodes 2, 4 have been proposed including a combination of implicit hyperbolic functions (see T. Y. Yang, Improved Uniform-Field Electrode Profiles for TEA Laser and High-Voltage Applications, The Review of Scientific Instruments, vol. 41, no. 4 (April 1973); G. J. Ernst, Uniform-Field Electrodes with Minimum Width, Optics Communications, vol. 49, no. 4 (Mar. 15, 1984); G. J. Ernst, Compact Uniform-Field Electrode Profiles, Optics Communications, vol. 47, no. 1 (Aug. 1, 1983)), and as a solution of a system of ordinary differential equations (see E. A. Stappaerts, A Novel Analytical Design Method for Discharge Laser Electrode Profiles, Appl. Phys. Lett., 40(12) (Jun. 15, 1982)).
Typical approaches usually propose the electrodes 2, 4 to be identical, each having a uniform regular shape with a minimal gap between the middle portions of the electrodes 2, 4 and a gradually increasing gap away from the middle portions to the edges. During laser operation, the discharge will begin in these middle portions. The real width of the gas discharge is also less than the width of the electrodes 2, 4. For example, the discharge width might be 11 mm while the width of each electrode 2, 4 is around 30 mm. The actual discharge width depends on many factors including the gas mixture, the preionization technique used, the electrical circuitry and the static electric field distribution.
The outer portions of the electrodes 2, 4, although carrying little or no discharge current, contribute significantly to the electrical field distribution in the vicinity of the discharge area 6. The fact that the outer portions of the electrodes 2, 4 carry little or no discharge current may be used advantageously for other considerations in the design of the electrodes 2, 4. For example, the outer portions of the electrodes 2, 4 may comprise dielectric materials such as ceramics to thereby prevent parasitic discharge currents and to further restrict the discharge width (see H. Bucher and H. Frowein, Elektrode fur einen Gasentladungslaser, Deutsches Patent DE 4401892 A1 (Jul. 27, 1995)).
A known design choice (see U.S. Pat. No. 5,557,629 to Mizoguchi et al. and U.S. Pat. No. 5,535,233 to Mizoguchi et al.) is to provide at least one of the electrodes 2, 4 with an elliptical shape such that the outer surface satisfies the relationship:
[x/a ]2 +[y/b]2=1, 
      where    ⁢          xe2x80x83        ⁢    1     less than       a    b     less than   4.
Another technique disclosed in the ""629 and ""233 patents is shown in FIG. 5. In the design shown in FIG. 5, additional xe2x80x9ceasingxe2x80x9d electrodes 36 are positioned on either side of the main discharge electrodes 2, 4.
It is an object of the present invention to provide an efficient discharge unit for line narrowed excimer or molecular fluorine lasers, operating at high repetition rates, such as are used as illumination sources in microlithography applications.
It is also an object of the invention to provide a discharge unit wherein the discharge circuit design including the placement of peaking capacitors Cp exhibits a low inductivity.
It is a further object of the invention to provide a discharge unit wherein gas flow conditions are optimized such that the laser gas may flow rapidly and uniformly through the discharge area between the main electrodes.
In accord with the above objects, in a first aspect of the present invention, an electrode chamber of a laser for an excimer or molecular fluorine laser is connected with a gas flow vessel, and includes a pair of elongated main electrodes separated by a discharge area, and a preionization unit. The electrode chamber also includes a spoiler integrated with the chamber and spaced from each of the main electrodes. The spoiler is shaped to provide an aerodynamic gas flow through the discharge area. A spoiler unit may include a pair of opposed spoiler elements each integrated with the chamber on either side of the discharge area, wherein each spoiler element is spaced from the main discharge electrodes and shaped to provide an aerodynamic gas flow through the discharge area.
Also in accord with the objects of the invention, in a second aspect of the present invention, a laser for an excimer or molecular fluorine laser is provided including an electrode chamber having a pair of elongated main electrodes separated by a discharge area, and a preionization unit. In the electrode chamber, at least one main electrode includes a base portion and a center portion which may be a nipple protruding from the base portion. The nipple substantially carries the periodic discharge current such that the discharge width is reduced to the width of the nipple which may be significantly less than the discharge width which would be provided by an electrode comprising only the base portion. The curvature of the base portion may be similar to the curvature of gas flow through the discharge chamber to improve aerodynamic performance.
In a third aspect of the present invention, an electrode chamber of a discharge unit for an excimer or molecular fluorine laser in accord with the above objects is connected with a gas flow vessel and includes a pair of main electrodes and a preionization unit. A plurality of ribs connected to one of the main electrodes cross the gas flow preferably between the electrode chamber and the gas flow vessel. The ribs are separated by openings to permit gas flow and shaped to provide an aerodynamic flow of gases through the openings. The shape of the ribs provides a smooth and uniform gas flow between the gas flow vessel and the electrode chamber and thus a reduced aerodynamic resistance for the blower over conventional conducting ribs. The ribs preferably have widths which smoothly taper from the end which meets the gas flow to the opposite end. The ribs may be rounded and each end may have a different radius of curvature.
In a fourth aspect of the invention, an electrode chamber of a laser for an excimer or molecular laser is connected with a gas flow vessel, and includes a pair of elongated main electrodes separated by a discharge area, and a preionization unit. The electrode chamber includes a spoiler spaced from each of the main electrodes and positioned near a preionization electrode to thereby shield the preionization electrode from one of the main electrodes. The spoiler is also shaped to provide an aerodynamic gas flow through the discharge area. A spoiler unit may include a pair of opposed spoiler elements each positioned electrode on either side of the discharge area to shield one of two or more preionization electrodes from a main electrode, wherein each spoiler element is spaced from the main discharge electrodes and shaped to provide an aerodynamic gas flow through the discharge area.
In a fifth aspect of the invention, an electrode chamber of a laser for an excimer or molecular laser is connected with a gas flow vessel, and includes a pair of elongated main electrodes separated by a discharge area, and a preionization unit. The electrode chamber includes a spoiler shaped to reflect acoustical waves emanating from the discharge area into the gas flow. The spoiler is also shaped to provide an aerodynamic gas flow through the discharge area. A spoiler unit may include a pair of opposed spoiler elements positioned on either side of the discharge area shaped to reflect acoustical waves emanating from the discharge area into the gas flow vessel, wherein each spoiler element is shaped to provide an aerodynamic gas flow through the discharge area.
Combinations of two or more of the features described above and below are also anticipated in the present invention. For example, a discharge chamber in accord with one, more than one or all three of the above aspects would be in accord with the present invention.